Full coverage of a second generation (2G) or third generation (3G) network such as Global System for Mobile Communications (GSM) or Wideband Code Division Multiple Access (WCDMA) has been basically implemented.
With development of network technologies such as System Architecture Evolution (SAE) and Long Term Evolution (LTE) in a fourth generation (4G) network, and with development of an evolved High Speed Uplink Packet Access (HSUPA) and High Speed Downlink Packet Access (HSDPA) technology, and a worldwide interoperability for microwave access (WiMax) network technology, these 4G networks or these evolved 3G networks have covered some urban areas and traffic hotspot areas. In this way, in current communications networks, these 4G networks and these evolved 3G networks coexist with a 2G or 3G network.
In the 2G or 3G network, a circuit switched (CS) domain is used to bear session services such as voice calls, but in the 4G or evolved 3G network, a packet switched (PS) domain is used to bear session services.
Circuit switched fallback (CSFB) is that when processing a voice service, user equipment (UE) covered by an LTE network falls back to a CS domain network first, and processes the voice service in the CS domain network, thereby fulfilling an objective of reusing an existing CS domain device to provide a traditional voice service for a user in the LTE network.
In the prior art, when the UE supports only handover from the LTE network to a PS domain of the 2G or 3G network, the UE initiates a voice call after being handed over to the PS domain of the 2G or 3G network, and the entire voice call consumes a relatively long time, which leads to poor user experience.